Hydrothermal circulation of seawater through the oceanic crust profoundly influences lithospheric and oceanic evolution. The upper oceanic crust comprises an aquifer of global proportions, and sedimentation acts to hydrologically isolate the crustal aquifer from the overlying ocean. Thermally-significant exchange of fluids between the crustal aquifer and overlying ocean can only occur where seamounts or other basaltic outcrops penetrate the sediment blanket. Although much progress has been made in the last several decades in elucidating the nature of hydrothermal circulation in oceanic basement, many of the physical properties and processes remain poorly understood or inadequately quantified. This volume addresses several fundamental gaps in our understanding of ridge-flank hydrological processes, namely the roles of seamounts and basement outcrops is facilitating the exchange of fluid between the crustal aquifer and overlying ocean, the extents and rates of hydrothermal circulation in uppermost basement, the bulk crustal permeability distributions required to support the fluid fluxes, the thermal effects of sedimentation on measurements of seafloor heat flux.